1. Field of the Invention
This invention relates to an optical communication system, a pump light supplying method and a distributed Raman amplifying apparatus which supply pump light in such a manner to secure flatness of gain as a function of wavelength, safety of an operator and avoidance of optical damage, in the optical communication system where Raman amplification is performed in an optical transmission line.
2. Description of the Related Art
Recently, with the objective of constructing a future multimedia network, optical communication apparatus with ultra-long distance and large capacity is required. Research and development on wavelength-division multiplexing (hereinafter abbreviated to “WDM”) is carried on in order to realize the large capacity, because it has the advantages of utilizing efficiently the properties of broadband/large-capacity of an optical fiber and the like.
Especially in an ultra-long distance optical communication system, the WDM optical signal needs to be amplified since the WDM optical signal is attenuated while transmitting in the ultra-long distance.
A conventional optical communication system includes a transmitting station in which the WDM optical signal is generated by multiplexing/demultiplexing a plurality of optical signals with different wavelengths from each other, an optical communication line which transmits the WDM optical signal sent out from the transmitting station, and a receiving station which receives the transmitted WDM optical signal, and further, one or more repeater station which has a function of amplifying the WDM optical signal is provided at some midpoint in the optical transmission line, as necessary.
In the optical communication system like the above, waveforms of the respective optical signals are equalized due to nonlinear optical effect in the optical transmission line. In order to avoid the waveform equalization, it is effective to reduce optical power (optical intensity) of the WDM optical signal which is made incident on the optical transmission line. Meanwhile, when the optical power is reduced, an optical signal-to-noise ratio becomes worse. As the nonlinear optical effects, for example, self-phase modulation, cross-phase modulation, four-wave mixing, stimulated Raman scattering (hereinafter abbreviated to “SRS”), stimulated Brillouin scattering (hereinafter abbreviated to “SBS”) and the like are known.
For this reason, it is proposed using a distributed optical amplifying apparatus which uses the optical transmission line also as an optical amplifying medium, together with a concentrated optical amplifying apparatus which is provided in the repeater station. For example, effectiveness of the Raman amplification is reported in P. B. Hansen, A. Stentz, T. N. Nielsen, R. Espinodola, L. E. Nelson, A. A. Abramov, uDense wavelength-division multiplexed transmission in “zero-dispersion” DSF by means of hybrid Raman/erubium-doped fiber amplifier” (OFC/100C '99), PD8, 1999, and N. Takachio, H. Suzuki, H. Masuda, and M. Koga, “32*10 Gb/s distributed Raman amplification transmission with 50-GHz channel spacing in the zero-dispersion region over 640 km of 1.55-μm dispersion-shifted fiber” (OFC/100C '99), PD9, 1999.
Moreover, in the Official Gazette of the Japanese Unexamined Patent Application No. Hei 03-013836, a method for Raman amplification is disclosed, in which test light is made incident on an optical transmission line and backward scattering is detected to obtain its loss in the optical transmission line, thereby performing the Raman amplification.
In the Official Gazette of the Japanese Unexamined Patent Application Publication No. Hei 10-073852, Raman amplification which broaden a wavelength band by using a plurality of pump lights with different wavelengths is disclosed.
Further, in the Official Gazette of the Japanese Unexamined Patent Application Publication No. Hei 10-022931, it is disclosed that a pump light source for Raman amplification is provided in a repeater station.
It should be mentioned that the optical amplifying apparatus can be classified into the concentrated optical amplifying apparatus and the distributed optical amplifying apparatus. The concentrated amplifying apparatus is an optical amplifying apparatus in which the optical amplifying medium and the pump light source are provided in a concentrated manner at one spot. For example, a laser diode amplifier and an optical fiber amplifier in which an optical fiber as the optical amplifying medium is wound around a bobbin or the like are well known. Meanwhile, the distributed optical amplifying apparatus is an optical amplifying apparatus in which the optical amplifying medium is laid over a fixed distance and the pump light source is provided at one side or both sides thereof. For example, there is an optical fiber amplifying apparatus. As the optical fiber amplifying apparatus, there are an optical fiber amplifier added with a rare earth element, an optical fiber amplifier using nonlinear scattering in the optical fiber and the like.
Physical process of amplifying the light is the same between the concentrated optical amplifying apparatus and the distributed optical amplifying apparatus, and the main difference is whether the optical amplifying medium is put together at one spot or distributed over the fixed distance. Further, the distributed optical amplifying apparatus has a characteristic that its optical amplifying medium also functions as the optical transmission line between the stations for transmitting the optical signals.
Moreover, as the nonlinear scattering, the SRS, the SBS and the like are known.
The SRS is a scattering occurring by an interaction with optical phonon of lattice vibration, which has a wide gain width and a large frequency shift. The SBS is a scattering occurring by an interaction with acoustic phonon of the lattice vibration, which has the narrower gain width and the smaller frequency shift than the SRS, but its gain coefficient is larger by more than two digits.
Characteristics of the optical fiber amplifier using the nonlinear scattering are that the common optical fiber such as an NZ-DSF and an SMF can be used, that a pump wavelength can be set to any amplification wavelengths, that the gain agrees with a polarization direction of the pump light, and so on. As the common optical fibers, for example, there are a dispersion-shifted optical fiber, a non-zero dispersion-shifted optical fiber (hereinafter abbreviated to “NZ-DSF”), a dispersion flat optical fiber, a single-mode optical fiber (hereinafter abbreviated to “SMF”) and the like.
It should be noted that all the above Official Gazettes disclose the Raman amplification, but do not specifically disclose how to control the pump light used for the Raman amplification. When it is disclosed, a complicated circuit is needed for the control.
Therefore, it is an object of the present invention to provide an optical communication system, a method for supplying pump light, and a distributed Raman amplifying apparatus in which the Raman amplification with substantially flat gain as a function of wavelength can be realized.
It is another object of the present invention to provide the optical communication system, the method for supplying pump light, and the distributed Raman amplifying apparatus which are preferable for securing the safety of operators who work with the optical communication system.
It is still another object of the present invention to provide the optical communication system, the method for supplying pump light, and the distributed Raman amplifying apparatus where occurrence of optical damage in the optical communication system is preferably prevented.